1. Field of the Invention
The present invention relates to an electric power steering apparatus for assisting the force required to turn the steering wheel of a vehicle.
2. Description of the Related Art
An electric power steering apparatus has been developed which uses a vehicle speed sensor to measure the traveling speed of the vehicle and a torque sensor to measure the steering torque applied at the steering wheel. In the electric power steering apparatus of this type, when the steering torque measured exceeds a predetermined dead zone, a steering assisting motor is driven by a driving current of the magnitude determined according to the measured steering torque and the measured vehicle speed, and the rotational force of the motor is used to provide power to assist the force required to steer the vehicle, thereby providing a comfortable steering feel to the driver. The steering assistance is controlled by a controller (hereinafter called the CPU). One such electric power steering apparatus employs a dual CPU configuration to assure the system safety against malfunctions, such as runaway, of the CPU.
The electric power steering apparatus with a dual CPU configuration has a main CPU for controlling the steering assisting force and a sub CPU for detecting failure of the main CPU by monitoring the inputs and outputs to and from the main CPU, so that upon detection of failure of the main CPU, the sub CPU initiates a fail-safe control, involving the stopping of the motor and so on, to secure the system safety.
In the dual CPU system, it is necessary to separately monitor the basic operation of each individual CPU. To achieve this, each of the main and sub CPUs is connected to a separate watchdog monitoring circuit which measures the frequency of watchdog pulses generated at a fixed repetition rate from the CPU connected to it and compares the measured frequency with a reference value to detect failure in the CPU operation. When any abnormality in the main or sub CPU operation is detected by the associated watchdog monitoring circuit, the fail-safe control is performed.
However, the above electric power steering apparatus having separate watchdog monitoring circuits for the main and sub CPUs has the problem of increasing the production cost because of the provision of the two watchdog monitoring circuits in the form of hardware in addition to the main and sub CPUs.